1. Field of the Invention
This invention relates to an optical recording medium capable of recording and erasing information by utilizing a change in the crystallographic structure of a recording layer and more particularly, to such an optical recording medium which is increased in track density and linear recording density.
2. Prior Art
Highlight is recently focused on optical recording media which can record information at a high density and erase the recorded information for rewriting. One typical rewritable optical recording medium is of phase change type wherein laser light is directed to the recording layer to change its crystallographic structure whereupon a concomitant change of reflectivity is detected. Optical recording media of the phase change type are of great interest because either recording or erasing may be chosen upon irradiation of a light beam simply by changing the intensity thereof, apparent overwrite recording is then possible with the use of a single light beam, and the drive unit has a simple optical system as compared with that of magnetooptical recording media.
Optical recording media of the phase change type can form small, sharp recorded marks owing to a so-called "self-sharpening effect" without a need for altering the drive unit optical system. More particularly, a recording light beam defines a spot on the recording layer surface. The beam spot has a higher cooling rate near its center where the temperature is higher and heat readily diffuses, but has a slower cooling rate near its periphery where the temperature is lower and thermal diffusion from the beam spot center comes over. This permits only the material near the beam spot center to be converted into an amorphous state by using recording light of an appropriate power. Consequently small, sharp recorded marks can be formed without reducing the wavelength of recording light, allowing for high density recording. This is often referred to as the self-sharpening effect. This is in contrast to magneto-optical recording media, for example, wherein no small recorded marks can be formed by utilizing differential cooling rates within a recording light irradiated spot because the recording layer is heated only to a low temperature below 200.degree. C. during recording.
Although optical recording media generally allow for high density recording, higher density recording is now required for recording of images. To increase the recording density per unit area, two approaches are contemplated, one being to narrow the pitch between recording tracks and the other being to reduce the spacing between record marks for increasing a linear record density. Optical recording media generally include grooves in the substrate surface for tracking purposes, and record marks are formed within the grooves. In the former approach of narrowing the track pitch, the grooves must also be reduced in width. If the optical system of the recording device is not altered, then the beam spot of recording light would have a larger diameter than the groove width and the resulting record mark would extend over an adjacent land between the grooves. If signals are reproduced from the thus recorded optical recording medium using reading light having a large beam spot diameter like the recording light, the influence of light reflected from the land would increase noise. A solution to this problem is to set the groove depth to 1/8 to 1/6 of the wavelength of reading light to induce a sufficient interference to minimize the influence of reflected light from the land. This solution, however, has a problem that deeper grooves lead to a loss of reflectivity, which in turn, leads to a loss of signal intensity, failing to provide high C/N. The latter approach of reducing the bit spacing has a problem that if the beam spot diameter defined in the recording layer by reading light is left unchanged from the conventional one, the beam spot would pick up information from a record mark disposed adjacent the spot in a scanning direction. The noise from the adjacent record mark leads to low C/N.
For preventing C/N drops upon reading of optical recording media having an increased track density or linear record density, the reading light should be reduced in beam spot diameter. This might be accomplished by reducing the wavelength of reading light or by increasing the numerical aperture of an objective lens in the optical pickup. Undesirably, both are technically difficult. Therefore, it is desired to improve C/N without resorting to reading light of shorter wavelength or an objective lens having a larger numerical aperture.
For recording of digital images containing a large quantity of information bits, not only high density recording, but also long time recording are necessary. For long time recording, the relative linear velocity of the medium relative to recording light must be low. If a longer record mark is consequently formed, an irradiation terminating or trailing zone can be sequentially, though slightly, heated due to the influence of an adjacent irradiated spot, resulting in slow cooling. Then the mark is not evenly converted into an amorphous state, failing to provide high C/N and good repetitive record properties. Under such circumstances, it is desired to prevent C/N drops in low-linear velocity recording.